关于SIP短信的问题

The requests for comments (RFC) states that it is not allowed to send a SIP message outside of session if the message size is at least 200 bytes less than the lowest maximum transmission unit (MTU) value found en route (usually this limit is 1300 bytes).
这句话怎么理解，为什么？

SIP消息小雨200字节时不允许从session中发出。200字节小于最低的MTU值（通常路由器中此值为1300字节）